1. Field of the Invention
This invention relates to a photodetector which is capable of detecting extremely weak light by using a photodiode, and more particularly to a photodetector which is applicable to environmental measurements, such as measurement of water droplets and dust particles, analyses of gases and trace materials in industrial fields, medical diagnoses, etc.
2. Description of the Related Art
As a photodetector of this kind, the present assignee has already developed a photodetector 31 shown in FIG. 3. This photodetector 31 is configured such that an extremely weak near-infrared light can be detected by using an avalanche photodiode 11 as a photodetector element, and includes a cooling block 2 constructed such that the avalanche photodiode 11 can be mounted on a pedestal arranged therein. Further, the photodetector 31 includes a power supply block 13 for generating bias voltage applied to the avalanche photodiode 11, a voltage detection block 32 for detecting the bias voltage of the avalanche photodiode 11 to output the detection signal indicative of the sensed bias voltage to the power supply block 13, a temperature sensor 33 attached to the pedestal in the cooling block 2 together with the avalanche photodiode 11, a temperature sensor block 34 for approximately detecting an temperature of the avalanche photodiode 11 based on a signal from the temperature sensor 33, a cooler 15 for cooling the pedestal in the cooling block 2 to thereby maintain the temperature of the avalanche photodiode 11 detected by the temperature sensor block 34 at a predetermined temperature, and a sensor block 36 for detecting an amount of an incident light impinging on the avalanche photodiode 11 based on a detection signal from the avalanche photodiode 11.
According to the photodetector 31 constructed as above, first, the cooler 15 cools the avalanche photodiode 11 to the predetermined temperature, and the power supply block 13 maintains the bias voltage of the avalanche photodiode 11 at a predetermined voltage. In this state, the temperature sensor block 34 detects a temperature of the avalanche photodiode 11 based on the signal from the temperature sensor 33, and the cooler 15 cools the avalanche photodiode by feedback control such that the temperature of the avalanche photodiode 11 detected by the temperature sensor block 34 becomes equal to the predetermined temperature. At the same time, the power supply block 13 is feedback-controlled such that the bias voltage detected by the voltage detection block 32 becomes equal to the predetermined voltage, thereby maintaining the bias voltage of the avalanche photodiode 11 at the predetermined voltage. Then, a signal light is blocked from impinging on the avalanche photodiode 11, and in this state, the sensor block 36 detects the amount of noise based on a dark current flowing through the avalanche photodiode 11. Next, a signal light is permitted to impinge on the avalanche photodiode 11, and in this state, the sensor block 36 detects the amount of signal light incident on the avalanche photodiode 11. Then, the sensor block 36 causes the amount of noise contained in the detected amount of the signal light incident on the avalanche photodiode 11, and the amount of noise detected when the signal light is blocked from impinging on the same, to cancel each other, thereby detecting the amount of the incident signal light itself.
As described above, in the photodetector 31, feedback control is carried out such that operating conditions, such as the cooling temperature and the bias voltage of the avalanche photodiode 11 become constant, and at the same time the amount of noise is eliminated by cancellation from the detected amount of the incident signal light to thereby reduce an error in the detection of the amount of the signal light.
However, the photodetector 31 has room for improvement in the following points: The avalanche photodiode 11 has detection characteristics very sensitive to changes in the cooling temperature in units of {fraction (1/100)}xc2x0 C. and changes in the bias voltage even in units of mV. On the other hand, heat enters the cooling block 2 from outside by way of an optical fiber cable connected to the avalanche photodiode 11, a cable for use in supply of the bias voltage, a cable for use in detecting the bias voltage, and a cable for use in detecting the temperature. In this case, if the amount of heat entering the cooling block 2 is constant, it is possible to hold the cooling temperature of the avalanche photodiode 11 constant to some extent by using the cooler 15, whereas if the ambient temperature outside the cooling block 2 changes, the amount of heat entering the cooling block 2 varies with this change. This causes as light change in the cooling temperature of the avalanche photodiode 11. Further, the sensitivity of the temperature sensor 33 per se varies with the lapse of time due to heat cycle etc. In addition, it is physically or mechanically difficult to bring the avalanche photodiode 11 into direct contact with the temperature sensor 33, and therefore, thermal resistance between them cannot be reduced to 0. As a result, the amount of change in temperature of the avalanche photodiode 11, and the amount of change in temperature detected by the temperature sensor 33 do not necessarily agree with each other. In view of the above problems, it is very difficult to control the temperature of the avalanche photodiode 11 itself to the order of accuracy of {fraction (1/100)}xc2x0 C.
Further, although the bias voltage of the avalanche photodiode 11 is feedback-controlled such that the same becomes equal to a predetermined voltage, it is very difficult to control the bias voltage such that it is not changed even in units of mV. Therefore, according to the photodetector 31, there can be an error in detection of the amount of an incident signal light or the amount of noise due to a slight change in the operating conditions of the avalanche photodiode 11, so that even if the amount of noise is cancelled out, there remains an error in the detected amount of the incident signal light. Further, since the voltage detection block 32 is connected to the avalanche photodiode 11, there is a fear that mixing of noise from the voltage detection block 32 degrades the detection accuracy of the photodetector 31. Therefore, there is a demand for enhanced detection accuracy on the photodetector 31.
Additionally, in the photodetector 31, it is required to feedback-control the bias voltage and the cooling temperature of the avalanche photodiode 11 with very high accuracy, so that the voltage detection block 32 and the temperature sensor 33 are required to have a high-precision detecting capability. This makes the photodetector 31 itself very expensive, and increases the size of the photodetector 31 against the demand of downsizing thereof. Therefore, there is also a demand for improvement in these points on the photodetector 31.
The present invention has been made in view of the above problems, and therefore, an object thereof is to provide a photodetector which is capable of attaining an enhanced detection accuracy and at the same time permits reduction in size and manufacturing costs thereof.
To attain the above object, the invention provides a photodetector including a photodiode for detecting an incident light, in a state of a predetermined bias voltage set thereto, and a cooler for cooling the photodiode to a predetermined cooling temperature, wherein an amount of the incident light on the photodiode is detected based on a detection signal from the photodiode.
The photodetector according to the invention is characterized by comprising a control block that adjusts at least one of the bias voltage and the predetermined cooling temperature such that a value of the detection signal from the photodiode generated in a state of the incident light being blocked from impinging on the photodiode is held within a predetermined tolerance range.
Here, the photodiode includes diodes for use in detection of light, such as an avalanche photodiode and a PIN photodiode.
According to this photodetector, the control block adjusts at least one of the bias voltage and the predetermined cooling temperature of the photodiode, thereby holding the value of the detection signal from the photodiode generated in a state of the incident light being blocked from impinging on the photodiode, within a predetermined tolerance range. This makes it possible to maintain constant operating conditions of the photodiode in the state of a signal light being blocked from impinging on the photodiode, with high accuracy, thereby enabling an extremely weak light to be sensed or measured with high accuracy. Further, it is possible to dispense with a voltage detection block which is conventionally required for high-accuracy control of the bias voltage, and high detection accuracy is also no longer required of the temperature sensor for detecting the cooling temperature of the photodiode. This make it possible to reduce the manufacturing costs and size of the photodetector.
Preferably, the photodetector includes a sensor block for converting the detection signal from the photodiode to a pulse signal, and counting pulses of the pulse signal to thereby detect the amount of the incident light, and the control block adjusts the at least one of the bias voltage and the predetermined cooling temperature such that a count value of the pulses counted by the sensor block in the state of the incident light being blocked from impinging on the photodiode is within a predetermined range, to thereby hold the value of the detection signal within the predetermined tolerance range.
According to this preferred embodiment, the count value of pulses of the pulse signal counted by the sensor block in a state of the incident light being blocked from impinging on the photodiode is maintained at a predetermined value or within a predetermined range. This makes it possible to digitally process or handle the value of the detection signal. Therefore, it is possible to carry out the measurement of a signal light more accurately and promptly than in a method of maintaining an analog amount, such as a current value, at a predetermined amount.
Preferably, the control block adjusts the bias voltage.
According to the preferred embodiment, the control block controls the bias voltage of the photodiode, whereby it is possible to control the operating conditions of the photodiode very promptly.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-81108, filed on Mar. 23, 2000, the disclosure of which is expressly incorporated herein by reference in the entirety.